The present invention relates to signal processors and, more particularly, to signal processors that are dynamically modifiable for optimal performance and reduced power dissipation.
In order to expand the dynamic range of a signal processing system, companding signal processors can be used. A companding signal processor uses an input amplifier to amplify or attenuate a signal before it is provided to the signal processor, and an output amplifier is used to amplify or attenuate the signal provided by the signal processor. A signal processor or signal processing circuit includes an active filter. The gain of the output amplifier is the inverse of the gain of the input amplifier, thus conserving the overall gain of the signal processor. Ideally, the gains of the input amplifier and output amplifier can be varied dynamically. A signal strength detector can be used to measure the strength of the input signal and provide a corresponding gain control signal. See Y. Tsividis, “Externally linear, time-invariant systems and their application to companding signal processors,” IEEE Transactions on Circuits and Systems II, Vol. 44, No. 2, February 1997. The gain control signal sets the amplification factors of the input amplifier and the output amplifier. However, this approach has the problem in that because the signal processor has memory, distortion in the output of the signal processor occurs whenever the amplification factors of the input amplifier and the output amplifier are changed.
The analog floating point technique addresses the problem of distortion in the output whenever the amplification factors change. See E. Blumenkrantz, “The analog floating point technique,” Proc. IEEE Symposium on Low Power Electronics, pp. 72-73, 1995. This technique avoids distortion by altering the state variables of the signal processor when the amplification factors change. However, implementation of the analog floating point technique is complicated, and is sensitive to parasitics and component mismatch. Accordingly, there is a need for circuits which expand the dynamic range of a signal processor without interrupting the output of the system or causing distortion.